competitive advantage must be actively deepened through isolating mechanisms because advantage that is not reinforced erodes
Competitive advantage is not a state -- it is a rate of change. An advantage that is not being actively deepened is being actively eroded by competition, imitation, and environmental change. Rumelt's "isolating mechanisms" are the structural features that prevent competitors from replicating an advantage: patents (temporary), switching costs (behavioral), network effects (demand-side scale), learning curves (supply-side scale), and proprietary information (knowledge asymmetry).
The critical insight is that isolating mechanisms must be investments, not inheritances. Network effects don't maintain themselves -- they require continued investment in platform quality and standards (Microsoft Windows' network effect eroded when web applications reduced switching costs). Learning curves only protect if the firm continues to move down them faster than entrants (Ford's Model T learning curve was overtaken by GM's flexible manufacturing). Patents expire. Switching costs decrease as competitors invest in migration tools.
The firm that treats its moat as self-sustaining will find it drained within a strategy cycle. The firm that invests its current advantage into deepening its isolating mechanisms compounds its position. Amazon's flywheel is the canonical example: lower prices leads to more customers leads to more sellers leads to more scale leads to lower costs leads to lower prices. Each cycle deepens the advantage, but only because Amazon reinvests margin into the flywheel rather than extracting it.
This connects to the broader pattern of compounding versus extraction. Any system -- firm, organism, civilization -- that extracts value from its current position without reinvesting in the mechanisms that created that position is on a declining trajectory. The advantage doesn't disappear suddenly; it erodes gradually until a single shock (a new competitor, a technology shift, a crisis) reveals that the moat was already gone.